1. Field of the Invention
The present invention relates to single acting, variable displacement fluid pressure vane pumps, such as for aircraft use, incorporating means for balancing cam bearing load, minimizing clocking forces, reducing friction and actuation loads.
Over the years, the standard of the commercial aviation gas turbine industry for main engine fuel pumps has been a single element, pressure-loaded, involute gear stage charged with a centrifugal boost stage. Such gear pumps are simple and extremely durable, although heavy and inefficient. However, such gear pumps are fixed displacement pumps which deliver uniform amounts of fluid, such as fuel, under all operating conditions. Certain operating conditions require different volumes of liquid, and it is desirable and/or necessary to vary the liquid supply, by means such as bypass systems which can cause overheating of the fuel or hydraulic fluid and which require heat transfer cooling components that add to the cost and the weight of the system.
2. State of the Art
Vane pumps and system have been developed in order to overcome some of the deficiencies of gear pumps, and reference is made to the following U.S. patents for their disclosures of several such pumps and systems: U.S. Pat. Nos. 4,247,263; 4,354,809; 4,529,361 and 4,711,619. Reference is also made to copending application Ser. No. 08/114,253, filed Aug. 30, 1993, the disclosure of which is hereby incorporated herein.
Vane pumps comprise a rotor element machined with slots supporting radially-movable vane elements, mounted within a cam member and manifold having fluid inlet and outlet ports in the cam surface through which the fluid is fed to the low pressure inlet areas or buckets of the rotor surface for rotation, compression and discharge from the high pressure outlet areas or buckets of the rotor surface as pressurized fluid.
Vane pumps that are required to operate at high speeds and pressures preferably employ hydrostatically (pressure balanced) vanes for minimizing frictional wear. Such pumps may also include rounded vane tips to reduce vane-to-cam surface stresses. Examples of vane pumps having pressure-balanced vanes which are also adapted to provide undervane pumping, may be found in the aforementioned copending application and in U.S. Pat. Nos. 3,711,227 and 4,354,809. The latter patent discloses a vane pump incorporating undervane pumping wherein the vanes are hydraulically balanced in not only the inlet and discharge areas but also in the seal arcs whereby the resultant pressure forces on a vane cannot displace it from engagement with a seal arc.
Variable displacement vane pumps contain a cam element which is adjustable relative to the rotor element, about a pivot point which is external to the cam chamber, in order to change the relative volumes of the inlet and outlet or discharge buckets and thereby vary the displacement capacity of the pump.
Among the problems encountered with single acting vane pumps with external cam adjustment pivots are the high actuation loads required to overcome the high friction cam sealing loads in order to move the cam member relative to the seals to adjust the displacement. Also, the external pivot support of the cam member results in high clocking loads or stresses on the cam member due to the differential bucket pressures on the externally-supported cam member.